Metal Solubility and Speciation. Metal Concentrations in Ore Fluids LA-ICPMS Fluid Inclusion Data Skarns Zn 5000 – 10,000 ppm Pb 500 – 5,000 ppm Ag 5.

Metal Solubility and Speciation

Metal Concentrations in Ore FluidsLA-ICPMS Fluid Inclusion Data

Skarns

Zn 5000 – 10,000 ppmPb 500 – 5,000 ppmAg 5 – 50 ppm

Ulrich et al. 1999 (Nature)Williams-Jones and Heinrich 2005 (Economic Geology)Klemm et al. 2008 (Mineralium Deposita)Samson et al., 2008 (Geology)

Porphyries

Cu 2000 – 10,000 ppmMo 500 – 1,500 ppmAu 80 – 800 ppb

Zinc content of crustal fluids

Zinc vs Lead in crustal fluids

Solvation (Hydration)

The polar nature of the water molecule causes separation of ionic species. The number of water molecules surrounding an ion (hydration number ) depends on the ionic radius.

Water molecules may be considered to be a simple electrical dipoles

Dielectric constant of water. Determined by creating an electrical field between two capacitor plates and measuring the voltage. The oriented dipoles create an internal field that opposes the external field. The dielectric constant is the ratio voltage in a vacuum over that in water.

The Dieletric Constant of Water

Properties of Water

Density Dielectric Constant

Ore Mineral Solubility as Simple Hydrated Ions

Complexation

Au SS

H

H

H

H

OH H

O

HH

O

H

H

O

H

H

O

H

H

O

2-Formation of soluble aqueous metal species, e.g. Au(HS)2

-

Potential Ligands for metal complexation

Ion-Pairing and Ligand availability

Dissociation constant of NaCl

Dissociation constant of HCl

Ionic (hard) Bonding

Transfer of electrons – electrostatic interaction

+_

Individual atoms with spherical electron clouds

Protons attract electron clouds and polarise each other

Covalent bond

Covalent (soft) bonding - polarisabilitySharing of electrons

Electronegativity and Chemical Bonding

• Ionic bonding – maximise electronegativity difference• Covalent bonding – minimise eletronegativity difference

Pearson’s Rules and Aqueous-Metal ComplexesHard cations (large Z/r) prefer to bond with hard anions (ionic bonding) and soft cations (small Z/r) with soft anions (covalent bonding)

Hard Borderline SoftAcids

Fe2+,Mn2+,Cu2+

Zn2+>Pb2+,Sn2+,As3+>Sb3+=Bi3+

H+, Na+>K+ Mg2+>Ca2+>Sr2+>Ba2+

Al3+>Ga3+

Y3+,REE3+ (Lu>La)Mo6+>W6+>Mo4+>W4+

Mn4+,Fe3+,U6+>U4+

BasesF-,OH-,CO3

2->HCO3-

NH3,SO42->HSO4

-

Acetate, Oxalate

Cl-

Au+>Ag+>Cu+ Hg2+>Cd2+

Pt2+>Pd2+

HS->H2SCN-,I->Br-

Gold solubility

1.5 m NaClP = 1000 bar

0.5 m KClpH buffered by K-feldspar-muscovite

SS = 0.01 m

A fO2 buffered by hematite-magnetite

B fO2 and fS2 buffered by Magnetite-pyrrhotite-pyrite

10

8

6

4

2

100 200 300Temperature ºC

log

βn

β2

β4

β1

β3

Ruaya and Seward (1986)

Stability of Zinc Chloride Species

log βn = log aZnCln2-n – log aZn2+ -nlog aCl- Zn2+

+ nCl- = ZnCln2-n

e.g., Zn2+ + 2Cl- = ZnCl20; β2

-4

-4 -3 -2 -1 0 1

log Cl (mol/Kg)

80

604020

80

604020P

erce

nt Z

n sp

ecie

s Zn2+

ZnCl+

ZnCl20ZnCl+

ZnCl42-

ZnCl3-

ZnCl42-

ZnCl20

350 ºC

150 ºC

β2

log

βn

16

14

12

10

β3 β4

100 200 3000

Temperature ºC

log

β11

3.5

3.0

2.5

Stability of Zinc Bisulphide Species

0 2 4 6 8 10

-5

-6

-7

-8

-9lo

g m

(Zn

) tota

l

150 ºCZ

n2+

Zn(HS)20

ZnS(HS)-

Zn(H

S) 3

-

pH

Zn2+ + nHS- = Zn(HS)n

2-n

Zn2+ + 2HS- = ZnS(HS)-

log βn = log aZn(HS)n2-n – log aZn2+ -nlog aHS-

log β11 = log aZnS(HS)- – log aZn2+ -2log aHS- -pH

Tagirov and Seward (2010)

Zn2+ + 2HS- = ZnS(HS)-

2 4 6 8 10 12

-3

-4

-6

-7

-8

-9

-2

-5

pH

mNaCl = 2 (12 Wt%)

mNaCl = 0.2 (1 Wt%)

mNaCl = 0.01

log

m Z

n to

tal

Zn-HS species

Zn-ClZn2+

300 ºC; 500 bar; ΣS = 0.05 m

2 4 6 8 10 12

-3

-4

-5

-6

-7

-8

-9

pH

mNaCl = 2 (12 Wt%)

mNaCl = 0.2 (1 Wt%)

mNaCl = 0.01

log

m Z

n to

tal

Zn-HS speciesZn2+

150 ºC; 500 bar; ΣS = 0.05 m

Zn-Cl

Tagirov and Seward (2010)

Relative Importance of Chloride and Bisulphide complexation

350

300

250

200

150

100

50

1 2 3 4 5 6 7 8 9 10

Tem

pera

ture

ºC

pH

10 ppm100 ppm1000 ppm10000 ppm

Solubillity of Sphalerite as a Function of Temperature and pH

2m NaCl0.01 mΣSSVP

(Based on data of Ruaya and Seward 1986; Tagirov and Seward, 2010)

Soluble

Insoluble

Gold solubility

T = 250 oCP = 500 bar

1 m NaCl

SS = 0.001 m

REE Complexation

REE forms very stable fluoride complexes, and less stable chloride complexes

The LREE are much more mobile than the LREE

Migdisov et al. (2009)

REE-fluoride solubility and REE Complexation

Association of HF at low pHand low solubility of REEPrecludes transport of REE as fluoride complexes.

Williams-Jones et al. (2012).

References

Williams-Jones, A.E., and Heinrich C.A., 2005, Vapor transport of metals and the formation of magmatic-hydrothermal ore deposits. Economic Geology 100: 1287-1312.

Eugster, H.P., 1986, Minerals in hot water. American Mineralogist, v.71, 655-673.

Crerar, D., Wood, S.M., Brantley, S., and Bocarsly, A., 1985, Chemical controls on solubility of ore-forming minerals in hydrothermal solutions. Canadian Mineralogist, v. 23, p. 333-352

Seward, T.M., and Barnes, H.L., 1997, Metal transport by hydrothermal fluids in Geochemistry of Hydrothermal Ore Deposits H.L. Barnes (ed), p. 235-285. John Wiley and Sons Inc.

Williams-Jones, A.E., \midisov, A.A. and Samson, I,M, 2012. The hydrothermal mobility of the rare earth elements – a tale of “ceria” and “yttria”. Elements, 8, in press.